Hives when doing outdoor exercise

I am a very active 34-year old woman living in Miami.  I have seasonal allegies to begin with, but I've noticed lately that when I do exercise outside, particularly running and cycling, that I develop a rash (or it could be hives) on my arms and sometimes on my legs.  

This rash is blotchy red and it stings and itches.  It was bad this morning when I went cycling, and I was also suffering from hayfever.  When I stop the exercise it goes away immediately. I also wore sunblock, though I don't think that is the cause.

I'm concerned, because I adore these activities, and I don't want to have to stop them (I hope I'm not allergic to my favorite sports!).  This rash is also very bothersome when I am doing the activity.

I don't always get this rash - when I am biking in other states or when I am working out indoors, I don't think I get it.

If you know what this is, or have any advice for me, I would greatly appreciate it. Thanks very much.  Randi M.

[National Jewish]

While it is not easy to make a diagnosis of the cause of a rash without seeing what it looks like, one possible cause of what you are describing is called "cholinergic urticaria". It behaves exactly as you have described. It is not a serious problem, but an annoying one. You might try antihistamines prior to activity. The reason for the rash is usually a histamine release with the heat generated by activity. The antihistamine works to block this.